There are many fields of manufacture in which the interior of a base body, such as a pipe or tube, or a segment thereof, is metalized over an ordinary metal such as steel with an expensive surface layer treatment or coating that is fused to the base metal in order to provide a finished, or partly finished, part or product that will respond to manufacturing specifications, but which is less expensive than making the entire body of the same material that the coating specifications require. Thus, parts such as the interior of pipes or tubes used to convey corrosive or abrasive fluids, liquids, slurries and the like, are frequently required to provide thereon an interior metalized surface of chromium, or chrome, or other special metal or metal alloy, that will either resist corrosion and wear or will provide a good bearing surface. In strings of pipe used in deep oil wells, for example, it is desirable that the interior surface of the pipe have resistance to corrosion and wear, so as to extend the time period during which a string of pipe will function before corrosion or abrasive failure causes disruption of oil production and consequent increase of costs. Similarly, strings of pipe which are used to transport concrete slurry from a source of supply to the site of use, must have a wear resistant inner surface in order to withstand the abrasion of the inner surface which is caused by the aggregate (sand, gravel, and crushed stone) which is mixed with the cement in the concrete slurry.
It has been long known that ordinary steels, except for leaded steels or resulphurized steels, may be chrome surfaced by plating or the like, to meet the specifications for desired strength of the part and provide the surface character specially required for exposure to a harsh environment in which the part is to be used.
However, chromium, for example, is a relatively expensive material, and the use of chromium in various chemical baths by which chrome plating may be effected, is environmentally undesirable, operationally difficult and expensive to control. Also, it is technically difficult to deposit a metalizing layer of any substantial thickness onto the interior surface of tubes or pipes, or segments thereof, that are to serve as the bearing surface of a bearing or journal element.
While metalizing the exterior surface of bars and rods avoids, to substantial extent, the undesirable environmental effects associated with chemical plating of such bodies, the mechanical metalizing techniques previously employed in metalizing such bars and rods have usually used an open flame torch that burns fuel gases, such as acetylene, propane, or the like in the presence of oxygen, to both preheat the body surface to an elevated temperature and to heat the surface application material, which is initially in powder form, to a temperature at which the powdered coating material will become at least partially molten and fuse onto the base material of the body. These prior art metalizing techniques have not been wholly successful for economically metalizing the exterior of tubes, since the heat of a torch will frequently burn through the wall of the tube. It will be understood that such prior art metalizing techniques also generally are not successful in metalizing the interior of elongated tubes and pipes, since access to the interior of such elongated bodies with an open flame torch is very difficult, if at all possible.
The problems with such a prior technique for metalizing exterior surfaces are that there is both lack of accurate control of the thickness of the layer of the surface application material to the underlying body, and resultant lack of uniformity of the thickness of the layer that is applied by open torch heat. Furthermore, the minimum thickness of the layer of applied material usually obtained by metalizing with an open flame torch working with powdered metal, is about 0.008 inch, and the maximum thickness of a layer of applied metal is about 0.015 inch, both of which thickness values are frequently much greater than the thickness of the applied material layer which is required to be supplied to meet the performance specifications for the metalized part, and this substantially increases the cost of manufacture.
A further problem is that when using fine particles of metalizing materials to form a fused surface on an underlying body, the torch heat intensity is frequently so great that it vaporizes or burns away a substantial quantity of the finest particles of the metalizing material, thereby resulting in loss of the coating material and economic waste. Still another problem is that, in the event a thick layer of metalizing is required to be deposited, there is insufficient control over the thickness of metal being deposited and, therefore, maintaining the concentricity of the inner surface of a metalized sleeve or journal is difficult, and machining or other expensive finishing operations must be resorted to in order to obtain a high degree of concentricity of the innermost surface of an arcuate part that has been metalized.
Other techniques are also available for metalizing with a vapor, either in an inert atmosphere or under vacuum. Such processes include chemical vapor deposition and physical vapor deposition, as by evaporation, ion plating, and sputtering. The products of these processes are coatings and free-standing shapes such as sheet, foil and tubing of thicknesses ranging from 20 nm to 25 mm. However, these processes do not lend themselves readily to the metalizing of the internal surface of long lengths of pipe or tubing.
An improved method of metalizing the interior of metal bodies is disclosed in U.S. Pat. No. 4,490,411 to Feder, which discloses an apparatus and method for metalizing the interior of pipes or tubes using powdered metal. The base metal pipe or tube which is to be internally metalized is moved axially while simultaneously being rotated at a relatively high rpm. A first preheat means, preferably comprising an induction heater, heats a portion of the pipe and its interior to a first elevated temperature, and the particles of the metalizing powder are deposited into the interior of the pipe to be heated to the first elevated temperature. The rotation of the pipe distributes the fluidized particles into laminae which under further influence of centrifugal forces, automatically distributes the semi-fluidized particles effectively. The fluidized metalizing material is bonded together and to the body substrate by application of a second induction heat at a higher temperature at which the bonding then occurs between the laminae of the metalizing material and between the metalizing material and the base material of the tube or pipe. Preferably, the process is performed in the presence of a non-oxidizing gas such as preheated nitrogen, neon, or argon.
Two means are disclosed for delivering the metalizing powder to the interior of the pipe to be metalized. In on embodiment, the metalizing powder is conveyed to the interior of the pipe by means of a cantilevered boom or supply-support tube through which the metalizing powder, entrained in a stream which includes a pressurized non-oxidizing gas, is delivered in the form of a spray or shower from a nozzle in the interior of the pipe at a station located laterally or axially between the two electrical induction heating coil means, with the first such induction heating means being a preheater and the second induction heating means being the metalizing heater for accomplishing the metal fusion. In the second embodiment, an elongated auger tube and concentric auger are utilized for delivering metalizing powder to the desired point of discharge between the first induction coil and the second induction coil.
Although the method and apparatus embodiments of U.S. Pat. No. 4,490,411 are capable of producing internally metalized pipe of acceptable quality, the patent teaches a method and apparatus for supporting, rotating, and axially advancing the pipe which is being metalized where these three functions are all accomplished by a single mechanism. This mechanism comprises a set of angular pipe-rotating means which are shown in FIG. 1 of the patent adjacent the left hand of the drawing. The means include a pair of pipe engaging drive rollers 14 located on opposite sides of the pipe 10 which is being coated, and frictionally engaging the pipe 10 in part below the midheight of the pipe so that the roller engagement with the pipe also serves as a support. The rollers are driven by any convenient well known means, such as electrical motors 16, at a high speed and selective speed control may be effected by techniques well known in the art. The direction of rotation of the angled drive rollers is indicated by the arrows 18 in FIG. 1 and they bring about the rotation of the pipe 10 and simultaneous axial movement of the pipe 10 as indicated generally by the spiral or helical arrow 20 which is shown associated with pipe 10 in FIG. 1. Because the rollers 14 are angularly oriented on the cylindrical surface of the pipe, they simultaneously rotate the pipe and axially advance the pipe through the heating zone.
It has been determined that this method and apparatus technique may create a potential problem, since the single unit is simultaneously rotating and pushing the pipe through the heating zone. Because the heating zone imposes sufficient heat to melt and fuse the powdered metal coating, the wall of the pipe 10 will approach a state of plasticity, so that the combination of rotation and axial advancement of the pipe by the single roller assembly can cause physical distortion and damage to the pipe at the zone of plasticity due to the simultaneous imposition of torsional and compressive forces applied by the angled roller assembly.
In order to avoid this potential problem, a coating apparatus has been fabricated and operated which comprises a plurality of first rotatable rollers mounted on a first rotatable shaft having an inlet and an outlet end, and a plurality of second rotatable rollers mounted on a second rotatable shaft having an inlet end adjacent the inlet end of the first rotatable shaft, and having an outlet end adjacent the outlet end of the first rotatable shaft. Each second roller is spaced from and adjacent to a corresponding first roller in a paired relationship to thereby define a gap of fixed dimension between each pair of adjacent first and second rollers. A single heating means which is operable to melt and fuse the powdered metal coating to the inside surface of a rotating pipe is located proximate the outlet ends of the first and second shafts. Preferably, the single heating means is an induction heater containing one or more induction coils for heating a rotating pipe as it passes through the center of the coils.
A plurality of third rollers mounted on a third rotatable shaft having an inlet end proximate the induction heating apparatus and having an outlet end spaced from the heating apparatus is paired on the discharge side of the induction heating apparatus with a plurality of fourth rollers mounted on a fourth rotatable shaft having an inlet end adjacent the inlet end of the third rotatable shaft, and having an outlet end adjacent the outlet end of the third rotatable shaft. Each fourth roller is spaced from and adjacent to a corresponding third roller in a paired relationship to thereby define a gap of the fixed dimension between each pair of adjacent third and fourth rollers.
A nesting groove is located above the gap between each pair of rotatable rollers and between the adjacent portion of the upper surfaces of each pair of rotatable rollers. A first motive means is typically utilized for rotating the first and second rotatable shafts and the mounted first and second rollers to thereby rotate an elongated tubular body, such as a pipe or tube, within the nesting grooves of adjacent pairs of first and second rollers, and a second motive means is typically utilized for rotating the third and fourth rotatable shafts and the mounted third and fourth rollers to thereby rotate the elongated tubular body within the nesting grooves of the adjacent pairs of the third and fourth rollers.
A separate individual axial advancing means is used for longitudinally advancing the rotating elongated tubular body sequentially along the nesting grooves between adjacent pairs of first and second rollers, through the induction heating means, and sequentially along the nesting grooves between adjacent pairs of third and fourth rollers. Finally, the means for internally coating the elongated tubular body is a spray head which is located within the heating element for depositing the particulate coating material on the inside surface of the pipe.
While this modified apparatus has been successful in eliminating the problem which may be encountered by use of the angular rotational apparatus disclosed in U.S. Pat. No. 4,490,411 of distorting and damaging the rotating and axial advancing pipe in the heated region of plasticity, a new and different problem has been encountered with the modified apparatus.
It has been found that as the heated rotating pipe is discharged from the induction heating means, there is an acute problem which arises due to the presence of a magnetic field which is imposed upon the hot pipe as it is advancing along the sequence of nesting grooves between the third and fourth rollers on the discharge side of the heating unit. This magnetic field typically becomes so intense that the rotating hot pipe may be drawn into the induction heater at a rate which is faster than the axial advancing means is pushing the rotating pipe. When this occurs, the residence time of the hot rotating pipe within the induction heating coil may become insufficient to allow the proper amount of sprayed metal powder to reach the internal surface of the pipe, thereby causing insufficient coating thickness to be fused to the inside surface of the pipe in at least some regions of the pipe bore.
Accordingly, it is an object of the present invention to provide a method and apparatus for internally coating the surfaces of elongated metal bodies, such as tubes and pipes, without encountering wide variations in the coating thickness on the internal surface of the metal bodies.
It is another object of the present invention to provide a method and apparatus for internally coating a metal pipe or tube without encountering wide variations in the speed of the rotating tube or pipe as it is passed through an induction heating apparatus.
It is also an object of the present invention to provide a method and apparatus for internally coating a metal tube or pipe without encountering an excessive speed of advance of the tube or pipe as it is passed through an induction heating apparatus while rotating and longitudinally advancing along the sequence of nesting grooves between the adjacent pairs of rollers on the outlet side of the induction heating apparatus.
It is a further object of the present invention to provide a method and apparatus for internally coating a rotating metal tube or pipe at a substantially constant speed of advance which is sufficient to cause the rotating and longitudinally advancing metal tube or pipe to achieve a substantially uniform coating thickness which is substantially uninfluenced by any magnetic field which may be imposed upon the advancing pipe as it is passed through an induction heating coil.
These and other objects of the present invention, as well as the advantages thereof, will become more clear to those skilled in the art from the disclosure which follows.